Osteochondromas are a relatively common imaging finding, accounting for 10-15% of all bone tumours and ~35% of all benign bone tumours. Although usually thought of as a benign bone tumour, they may be thought of as a developmental anomaly. They are frequently asymptomatic and have very low malignant potential if sporadic and solitary.

Terminology

Osteochondromas are distinct from other exostoses because of their cartilaginous cap covered by periosteum ^8,10. The term (osteo)cartilaginous exostosis is no longer recommended ⁸.

Epidemiology

Osteochondromas develop in childhood, can grow during skeletal maturation, but once formed, regression or resolution is rare ^15-17. Occurrence or progression after puberty is unusual ¹⁸. They are usually an asymptomatic incidental finding ⁸.

They are most commonly sporadic, but can be part of:

• hereditary multiple exostoses (HME a.k.a. diaphyseal aclasis): 15% of osteochondroma patients ⁸

• Trevor disease (a.k.a. dysplasia epiphysealis hemimelica)

• sometimes arise after fractures, trauma, radiation, and haematopoietic stem cell transplantation²¹

Malignant transformation occurs in the cartilage cap, and is uncommon in sporadic solitary osteochondromas (~1%), whereas in the setting of hereditary multiple exostoses the rate is much higher (5-25%) ⁵.

Risk factors

• childhood total body irradiation ⁸

Diagnosis

Diagnostic criteria according to the WHO classification of soft tissue and bone tumours (5th edition) ⁸:

• essential

  • cartilaginous cap <2 cm

  • growth-like architecture (in children) or extensive calcification as age increases

  • underlying stalk with medullary and cortical continuation with underlying bone

Clinical presentation

Symptomatic presentation is either due to mechanical effects of the lesion, fracture, or malignant transformation. Mechanical symptoms include ^8,9:

• hard palpable lump (most common)

• impingement upon nearby structures

  • nerve compression (up to 25%)

  • vascular compression

  • reactive myositis

  • spinal cord compression

• bursal formation and bursitis

• osteoarthritis from secondary joint deformity

• fracture post-trauma: most commonly through the neck of pedunculated lesions at the knee

Ongoing growth and or pain after skeletal maturity has been reached is suspicious for malignant degeneration. 

Pathology

Osteochondromas are considered a chondroid neoplasm and are primarily a part of the growth plate which separates and continues growing independently, without an associated epiphysis, usually away from the nearby joint. The medullary cavity is continuous with the parent bone, and hyaline cartilage caps them. Osteochondromas can be congenital or occur as a result of previous trauma to the growth plate, including previous irradiation ^3,4.

Location

They most commonly arise from the appendicular skeleton, especially around the knee ³:

• lower limb: 50% of all cases ³

  • femur (especially distal): most common: 30%

  • tibia (especially proximal): 15-20%

  • less common locations: feet, pelvis

• upper limb

  • humerus: 10-20%

  • less common locations: hands, scapula. They are the commonest tumours of the scapula, accounting for 4.6% of all bone tumours ^11,12.  

• spine: the posterior elements of the spine are an uncommon but not rare site for these tumours

• ribs: rare location, but can have serious complications including spontaneous haemothorax, pneumothorax, diaphragmatic rupture, recurrent empyema, vascular and nerve impingement ^13,14

Radiographic features

Plain radiograph

An osteochondroma can be either sessile or pedunculated and is seen in the metaphyseal region typically projecting away from the epiphysis. There is often associated broadening of the metaphysis from which it arises. The cartilage cap is variable in appearance. It may be thin and difficult to identify, or thick with rings and arcs calcification and irregular subchondral bone. 

New cortical irregularity or continued growth after skeletal maturity has been reached, as well as frankly aggressive features (e.g. bony destruction, large soft tissue component, metastases) are all worrying for malignant transformation. 

CT

CT demonstrates the same findings as on radiograph but is better able to demonstrate medullary continuity and the cartilage cap. Septal enhancement may be seen between lobules of cartilage ²⁰.

Ultrasound

Ultrasound is able to demonstrate the cartilage cap very accurately as a hypoechoic region bounded by bone on its deep surface and muscle/fat superficially.

MRI

MRI is the best imaging modality to assess cartilage thickness (and thus assessing for malignant transformation), the presence of oedema in bone or adjacent soft tissues, and visualising neurovascular structures in the vicinity.

The cartilage cap of osteochondromas appears the same as cartilage elsewhere, with intermediate to low signal on T1 and high signal on T2 weighted images. 

A cartilage cap of over 1.5 cm in thickness after skeletal maturity is suspicious for malignant degeneration, while the cartilage cap maybe seen up to 3 cm in thickness in young patients ³.

With intravenous gadolinium administration, enhancement of benign lesions is normally seen in the peripheral fibrovascular tissue covering the cartilaginous cap ²⁰. The cartilaginous cap itself should not enhance ²⁰. In contrast, chondrosarcoma may demonstrate septal enhancement between lobules of cartilage ^3,19,20.

Nuclear medicine

During growth, osteochondromas demonstrate increased uptake on bone scans, but with time they become no more active than normal bone. Presence of increased activity in adulthood should raise the possibility of a complication (e.g. fracture, malignancy).

Treatment and prognosis

As they represent normal growth of a growth plate in an abnormal direction, they often stop growing with the rest of the skeleton ³. 

In most instances, no treatment is required. The osteochondroma can be locally resected for cosmetic or mechanical reasons.

If malignant transformation occurs (~1% in solitary osteochondromas and ~5-25% with hereditary multiple exostoses) then the resultant chondrosarcoma is usually of low grade (67-85% of cases), and surgery is usually curative (70-90%) ^3,5. Limb-sparing wide local excision usually suffices. The features associated with sarcomatous transformation can be remembered by the mnemonic GLAD PAST ¹:

• growth after skeletal maturity

• lucency (new)

• additional scintigraphic activity

• destruction (cortical)

• pain after puberty

• and

• soft tissue mass

• thickened cartilage cap greater than 1.5 cm

Local recurrence is seen in both benign and malignant lesions, due to spillage of cartilage cells into the resection bed. Rates are estimated at 2% and up to 15% respectively ³. 

Differential diagnosis

Imaging differential considerations include:

• hands: bizarre parosteal osteochondromatous proliferation (BPOP): arises from the bone cortex and lacks medullary continuity

• humerus: supracondylar spur: projects towards the elbow joint

• pelvis

  • pelvic digit

  • rectus femoris heterotopic ossification

  • myositis ossificans

  • AIIS avulsion injury

• malunited fracture